1. Field of the Invention
Generally, the invention relates to urethane prepolymers used to make polyurethane foams, and more specifically, urethane prepolymers used to make water absorbent polyurethane and high water capacity foams.
2. Description of Prior Art
The prior art discloses various compositions and methods used to make polyurethane prepolymers and water absorbent polyurethane foams resulting from such prepolymers. Recently, there have been attempts to achieve highly water absorbent foams which also exhibit dimensional stability, e.g. ability to retain shape and rigidity, and hold the absorbed water even under pressure. See Non Wovens Market and Fiber Structures Report, p. 3, July 14, 1989. This ability to retain water under pressure also reflects the foam's load bearing characteristics. Further, materials possessing these characteristics have also been recognized as useful in products such as surgical dressings, diapers, bedpads, etc. See U.S. Pat. No. 3,939,123 to Matthews et al, Col. 1, lines 13-23.
One known method for making prepolymers for water absorbent foams is to make the prepolymer with hydrophilic components such as polyoxyethylene oxide containing polyols. See U.S. Pat. No. 3,903,232 to Wood and Frisch. However, when wet, such foams undergo a large amount of swelling and water plasticization of the polymer matrix. Further, Wood and Frisch disclose that they can compress their foams to form soft dense materials and upon exposure to water, regenerate them to the original volume, e.g. fifteen to twenty times expansion. See Column 2, lines 38-42 and lines 52-57. Because of this expansion and the accompanying swelling and plasticization, the Wood et al foams lose their structural rigidity and release substantial amounts of water when handled. Thus their ability to hold water is diminished. Also see U.S. Pat. No. 4,160,076. (hydrophilic polyurethane sponge which absorbs six times its weight in water).
More recent methods achieve a water absorbent foam by incorporating hydrophilic components into the prepolymer. For instance, U.S. Pat. No. 4,738,992 to Larson and Hedrick discloses the incorporation of ionic hydrophilic functional groups, such as sulfonate groups, into a polyurea/urethane, isocyanate terminated prepolymer used to make water absorbent sponges. Of course, the hydrophilicity of the functional groups enhances the overall water absorption properties of the foam. See also U.S. Pat. No. 4,638,017 to Larson and Hedrick.
The addition of hydrophilic polycarbonyl moieties, e.g. those found on acrylates, to the polyurethane structure of a foam represents a similar approach to Larson and Hendrick's method. See U.S. Pat. No. 4,725,628 to Garvey et al. See also U.S. Pat. No. 4,725,629 to Garvey and Pazos.
Water absorbent foams also have been made using hydrophobic materials. For instance, U.S. Pat. No. 4,377,645 to Guthrie and Arquette discloses foaming prepolymers wherein isocyanate capped prepolymers are prepared from hydrophobic methylene diphenyl isocyanate, (MDI), trimethyolpropane and a polyoxyalkylene diol. While the foam resulting from the MDI terminated prepolymer only swells 19% by volume, it is only able to hold a little over 20 times its weight in water. These foams are suitable for household sponges and substrates for flocked wall coverings, as well as suitable for flexible foams used in medical or personal care items. See also U.S. Pat. Nos. 4,384,050 and 4,384,051 to Guthrie.
To enhance a foam's dimensional stability, additives have been used in making water absorbent foams. Typically, additives are also used to improve the durability and the load bearing characteristics of the foam. A popular method for achieving these characteristics is the addition of reinforcing agents. For instance, U.S. Pat. No. 4,314,034 to Fulmer and Vollmerhausen discloses the addition of polyester fibers to provide structural rigidity. See also U.S. Pat. No. 4,201,846 to Kehr et al. (polyvinyl alcohol fibers used for reinforcement in polyurethane foams) and U.S. Pat. No. 3,959,191 to Kehr and Marans (addition of solid materials such as fibrous materials to improve load bearing characteristics.)
However, the prior art approaches for achieving water absorbency and dimensional stability have disadvantages. For example, the super absorbent foams disclosed by Larson and Hedrick are made by the attachment of ionic groups to the urethane prepolymer and thus involves additional chemical processing to achieve the desired water absorbent properties. Garvey et al's use of the methacrylates results in foams which undergo undue swelling when wet.
The foams made from MDI and reinforcing agents, have relatively good structural stability, but do not have sufficient water capacity characteristics because the reinforcing agents increase foam density and deplete void space. As mentioned above, MDI-based foams without reinforcing agents were only able to hold water in an amount up to about twenty times their weight.